US 3808572 A
A transformer type heater for the bimetal trip element of a circuit breaker is provided with an auxiliary magnetic structure including means for diverting heating energy from the transformer secondary to a power dissipating means to thereby control the heating energy acting to heat the bimetal. The power dissipating means consists of a lumped resistor, the value of which may be controlled or set very readily within required tolerances, thereby facilitating the production of an accurately calibrated circuit breaker thermal trip means.
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Description (OCR text may contain errors)
United States Patent Gaskill [451 Apr. 30, 1974 INDUCTIVE HEATING MEANS FOR THERMAL TRIP DEVICE  Inventor: George Gaskill, Hatboro, Pa.
 Assignee: I-T-E Imperial Corporation, Spring House, Pa.
 Filed: July 12, 1973  Appl. No.: 378,416
52 us. on. 337/106  Int. Cl. H0lh 61/02  Field of Search 337/106, 40
 References Cited UNITED STATES PATENTS 2,939,929 6/1960 Hobson, Jr 337/106 Primary ExaminerHarold Broome Attorney, Agent, or FirmOstrolenk, Faber, Gerb &
Soffen  ABSTRACT A transformer type heater for the bimetal trip element of a circuit breaker is provided with an auxiliary magnetic structure including means for diverting heating energy from the transformer secondary to a power dissipating means to thereby control the heating energy acting to heat the bimetal. The power dissipating means consists of a lumped resistor, the value of which may be controlled or set very readily within required tolerances, thereby facilitating the production of an accurately calibrated circuit breaker thermal trip means.
6 Claims, 2 Drawing Figures PATENTEDAPR 30 974 PRIOR ART INDUCTIVE HEATING MEANS FOR THERMAL TRIP DEVICE This invention relates to circuit breakers in general and relates more particularly to transformer type means for heating the thermal element of an automatic trip device.
Automatic circuit breakers constructed to operate at relatively high values of continuous current and having the capability of interrupting extremely high values of fault currents are often provided with thermal or time delay tripping means, including a bimetal that is heated by being conductlvely mounted to the secondary of a transformer whose primary consists of a circuit breaker main conductor. This type of construction is illustrated in U. S. Pat. No. 2,939,929, issued June 7, 1960, to C. F. Hobson Jr., for an Electric Circuit Breaker.
It has been found that with prior art constructions conductive heating means for the thermal elements of circuit breakers must be constructed to very close tolerances to obtain properly controlled heating of the bimetal. Close tolerances are particularly important insofar as lengths of air gaps in the magnetic frame of the heating means are concerned. Accurate control of bimetal heating must be maintained so that the bimetal element is heated within its most sensitive range, and the rate of heating must be high enough to produce thermal tripping action within the time required to maintain a specified tripping characteristic. The heat transfer characteristics of the heater-thermally responsive element combination and the magnitude of circuit breaker current determine total deflection and time rate of deflection for the thermally'responsive element.
If interchangeability of inductively heated theremal trip units is desired, it is necessary to closely control assembly tolerances from unit to unit in order to maintain calibration accuracy. In accordance with the instant invention, means are provided to facilitate calibration of an inductively heated thermally responsive trip means thereby, to a large extent, negating effects of component manufactuiing and assembly tolerances from unit to unit. As will hereinafter be seen, trip unit calibration is achieved very simply and accurately merely by substitution of a lump resistor or by utilizing a variable resistor.
Accordingly, a primary object of the instant invention is to provide a novel, improved means for inductively heating a circuit breaker thermal trip means.
Another object is to provide an inductive heating means of this type constructed so as to facilitate the maintenance of calibration accuracy.
Still another object is to provide an inductive heating means of this type in which calibration is facilitated.
A further object is to provide an inductive heating means of this type in which calibration is achieved by merely changing a resistance value.
These objects as well as other objects of this invention will become readily apparent after reading the following description of the accompanying drawings in which:
FIG. 1 is a perspective illustrating a prior art construction of a time delay tripping bimetal and its inductive heating means.
FIG. 2 is a perspective, partly in schematic form, illustrating a circuit breaker having a bimetal inductive heating means constructed in accordance with teachings of the instant invention.
Now referring to the figures. In FIG. 1 bimetal strip 11 constitutes the thermally responsive element of a circuit breaker-time delay trip means. One end of bimetal ll is secured to one leg of shading coil 12 in heat conducting relationship therewith so that heat generated by current flowing in coil 12 will heat bimetal 11 causing the free end 16 thereof remote from coil 12 to deflect. Coil 12 is wound about the horizontal web portion of inverted, generally U-shaped laminated core element 13, whose arms are aligned with the upwardly extending arms of the other U-shaped, laminated core element 14 of the magnetic frame. Main circuit breaker conductor 15 extends through magnetic frame l3, l4 and constitutes a single turn primary for the transformer including magnetic frame l3, l4 and secondary winding 12. 7
Thus, alternating current flowing through main conductor 15 generates magnetic flux in magnetic frame 13, 14, with this flux inducing current flow in secondary or shading winding 12. In turn, the current flowing in coil 12 causes heating thereof, with this heat being transferred by conduction to bimetal 11, causing free end 16 thereof to deflect, with the extent of the deflection being a function of bimetal temperature. In a manner well known to the art, a predetermined deflection of bimetal 11 will cause the latter to release a latch, such as element 24 in FIG. 2, causing a circuit breaker operating mechanism, such as 26 in FIG. 2, to trip.
Now referring to FIG. 2, wherein conductive heating means constructed in accordance with teachings of the instant invention is illustrated. Those elements common to both FIGS. 1 and 2 are given the same reference numerals in both figures.
In FIG. 2 shading coil 12 is also wound about one leg of rectangular loop-shaped auxiliary core 17 having energy diverting circuit 18 coupled thereto. Circuit 18 includes variable lumped resistor 19. Current flowing in shading coil 12 induces flux in auxiliary magnetic core 17, which in turn induces current to flow in secondary circuit 18 through resistor 19 thereof. In effect, power is being withdrawn from shading coil 12 and is being dissipated in resistor 19, so that less power is dissipated in shading coil 12, and as a result less heat is available for heating of bimetal 11.
As bimetal 11 heats, free end 16 thereof reflects to the right, with adjusting screw 21 carried thereby engaging ear 22 extending radially from trip bar 23. This causes clockwise pivoting of the latter, with radial latching extension 24 thereof moving clear of latchable arm 25 extending from contact operating mechanism 26. In a manner well known to the art, release of latchable member 25 causes contact operating mechanism 26 to operate movable contact arm 27 so that movable contact 28 carried thereby separates from stationary contact 29 mounted on main conductor 15.
Since the turns ratio between shading coil 12 and secondary circuit 18 is arbitrary, the value of resistor 19 may be made many orders of magnitude largerthan the resistance of shading coil 12, the latter being of very low resistance, so that resistor 19 may be of reasonable physical size. The number of turns in circuit 18 may be counted accurately, and it is a simple matter to produce a resistor of relatively high precision to use in secondary circuit 18. The resistance of shading coil 12 may also be controlled accurately. Accuracy and reliability of calibration depends upon the value of resistor 19, which, although shown as an adjustable resistor, may be an individually adjusted fixed precision resistor. Accordingly, variation in functional characteristics from unit to unit may be controlled relatively closely, so that the changing of a tripping range may be accomplished simply by changing a resistor in the secondary of the auxiliary magnetic structure.
Even through the instant invention has been described with auxiliary magnetic circuit 17, 18 mounted to the same shading coil 12 to which bimetal 11 is secured, it is noted that auxiliary circuit 17, 19 may be coupled to magnetic frame 13, 14 through a second shading coil (not shown).
Although there have been described preferred embodiments of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited not by the specific disclosure herein but only by the appending claims.
The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:
1. A circuit interrupter having automatic trip means comprising transformer means including a core structure, primary means and secondary means both magnetically coupled to said core, thermal trip means mounted in heat conducting relationship with respect to said secondary means, an auxiliary magnetic structure magnetically coupled to said core structure and including power dissipating means diverting heating energy from said secondary means to control the heating energy acting on the thermal trip means.
2. A circuit interrupter as set forth in claim 1 in which the power dissipating means includes a circuit having a power resistance therein.
3. A circuit interrupter as set forth in claim 2 in which the value of the resistance is adjustable to control the magnitude of heating energy diverted from said secondary means.
4. A circuit interrupter as set forth in claim 2 in which the auxiliary magnetic structure includes an auxiliary secondary means through which the circuit is coupled to the secondary means.
5. A circuit interrupter as set forth in claim 4 in which the resistance is lumped.
6. A circuit interrupter as set forth in claim 5 in which the value of the resistance is adjustable to control the magnitude of heating energy diverted from said secondary means.